Method and apparatus for making laminar plastic sheet material

Abstract

A tubular, laminar, plastic sheet material having a core layer of oriented plastic and two outer layers of approximately isotropic plastic is prepared in extrusion apparatus whose die block has three channels wider than they are thick, and merging at acute angles to form an extrusion nozzle. The channels are continuously supplied with molten resin compositions webs of which are superimposed on each other in area contact at the junction of the channels. They move as an initially fluid, but coherent, layered structure from the nozzle into an immediately adjacent duct of a cooling head in which the structure is cooled from a first temperature above the melting temperature of the highest-melting resin composition of the central layer to a temperature intermediate that melting temperature and the temperature at which the outer, lower melting layers become solid. The friction exerted on the fluid outer layers by the walls of the duct in the cooling head exerts a sufficient braking action on all three layers that the laminar structure may be drawn from the outer orifice of the cooling head under tension high enough to stretch the laminar structure to twice its initial length or more without failure in tension of the plastic material in the cooling head and the die nozzle. Only the central layer is solid at the outer orifice of the cooling head so as to become oriented while the still fluid outer layers remain isotropic.

What is claimed is: 
     
       1. A method of preparing a laminar plastic material which comprises: a. continuously and simultaneously moving a first elongated web of a first synthetic resin composition, a second elongated web of a second synthetic resin composition, and a third elongated web of a third synthetic resin composition in the direction of elongation thereof to a first zone and thence through a second zone to a third zone, 1. each web having two opposite longitudinal faces,   2. said second composition having a higher melting temperature than the melting temperatures of said first and third compositions,     
     
     
       3. said second composition being capable of being oriented when said second web is stretched longitudinally while in the solid state; b. superimposing respective longitudinal faces of said first and third webs in said first zone on the longitudinal faces of said second web under a pressure sufficient to combine said webs into a coherent layered structure, 1. said first zone being at a first temperature higher than the melting temperature of said second composition,   2. said structure being moved from said first zone to said second zone at a first linear speed, and being confined in each of said first and second zones under said sufficient pressure;     c. cooling said structure during movement through said second zone from said first temperature to a second temperature lower than the melting temperature of said second composition, but higher than the melting temperatures of said first and third compositions, 1. the rate of movement of said structure through said second zone being slow enough to cause solidification of said second composition in said second zone;     d. applying frictional braking force to said first and third webs in said second zone and transmitting said braking force from said first and third webs to said second web in said second zone;   e. cooling said structure in said third zone below the melting temperature of said first and third compositions;   f. applying longitudinal tension to said cooled structure in said third zone at a rate sufficient to move said structure from said second into said third zone at a second speed substantially greater than said first speed,   
     
     
       1. said braking force being sufficient to prevent failure of said second web in said second zone under the tension applied in said third zone, while permitting stretching of said structure during movement thereof from said second to said third zone under said applied tension, thereby orienting said second composition. 
     
     
       2. A method as set forth in claim 1, wherein said first and third compositions are substantially identical. 
     
     
       3. A method as set forth in claim 1, wherein said webs are tubular. 
     
     
       4. A method as set forth in claim 3, wherein said second composition essentially consists of polyamide and said first and third compositions essentially consist of olefin polymers. 
     
     
       5. A method as set forth in claim 1, wherein said pressure is maintained during said applying of longitudinal tension.